Journal ArticleSulfur dioxide in the mid-infrared transmission spectrum of WASP-39bPowell, Diana; Feinstein, Adina D; Lee, Elspeth_K H; Zhang, Michael; Tsai, Shang-Min; Taylor, Jake; Kirk, James; Bell, Taylor; Barstow, Joanna K; Gao, Peter; Bean, Jacob L; Blecic, Jasmina; Chubb, Katy L; Crossfield, Ian_J M; Jordan, Sean; Kitzmann, Daniel; Moran, Sarah E; Morello, Giuseppe; Moses, Julianne I; Welbanks, Luis; Yang, Jeehyun; Zhang, Xi; Ahrer, Eva-Maria; Bello-Arufe, Aaron; Brande, Jonathan; Casewell, S L; Crouzet, Nicolas; Cubillos, Patricio E; Demory, Brice-Olivier; Dyrek, Achrène; Flagg, Laura; Hu, Renyu; Inglis, Julie; Jones, Kathryn D; Kreidberg, Laura; López-Morales, Mercedes; Lagage, Pierre-Olivier; Meier_Valdés, Erik A; Miguel, Yamila; Parmentier, Vivien; Piette, Anjali_A A; Rackham, Benjamin V; Radica, Michael; Redfield, Seth; Stevenson, Kevin B; Wakeford, Hannah R; Aggarwal, Keshav; Alam, Munazza K; Batalha, Natalie M; Batalha, Natasha E; Benneke, Björn; Berta-Thompson, Zach K; Brady, Ryan P; Caceres, Claudio; Carter, Aarynn L; Désert, Jean-Michel; Harrington, Joseph; Iro, Nicolas; Line, Michael R; Lothringer, Joshua D; MacDonald, Ryan J; Mancini, Luigi; Molaverdikhani, Karan; Mukherjee, Sagnick; Nixon, Matthew C; Oza, Apurva V; Palle, Enric; Rustamkulov, Zafar; Sing, David K; Steinrueck, Maria E; Venot, Olivia; Wheatley, Peter J; Yurchenko, Sergei N<title>Abstract</title> <p>The recent inference of sulfur dioxide (SO₂) in the atmosphere of the hot (approximately 1,100 K), Saturn-mass exoplanet WASP-39b from near-infrared JWST observations^1–3suggests that photochemistry is a key process in high-temperature exoplanet atmospheres⁴. This is because of the low (<1 ppb) abundance of SO₂under thermochemical equilibrium compared with that produced from the photochemistry of H₂O and H₂S (1–10 ppm)^4–9. However, the SO₂inference was made from a single, small molecular feature in the transmission spectrum of WASP-39b at 4.05 μm and, therefore, the detection of other SO₂absorption bands at different wavelengths is needed to better constrain the SO₂abundance. Here we report the detection of SO₂spectral features at 7.7 and 8.5 μm in the 5–12-μm transmission spectrum of WASP-39b measured by the JWST Mid-Infrared Instrument (MIRI) Low Resolution Spectrometer (LRS)¹⁰. Our observations suggest an abundance of SO₂of 0.5–25 ppm (1<italic>σ</italic>range), consistent with previous findings⁴. As well as SO₂, we find broad water-vapour absorption features, as well as an unexplained decrease in the transit depth at wavelengths longer than 10 μm. Fitting the spectrum with a grid of atmospheric forward models, we derive an atmospheric heavy-element content (metallicity) for WASP-39b of approximately 7.1–8.0 times solar and demonstrate that photochemistry shapes the spectra of WASP-39b across a broad wavelength range.</p>Nature2024-02-2910582950Nature6268001979 to 9830028-0836https://doi.org/10.1038/s41586-024-07040-91945633National Science Foundation